This invention relates to nonwoven webs prepared from thermoplastic polymers and to products incorporating apertured nonwoven webs.
Apertured nonwoven webs of thermoplastic polymers are often used in the manufacture of disposable absorbent articles, including wipes, garments, and hygiene products. One typical use is as a topsheet in diapers, feminine hygiene products, incontinence garments, and the like. The topsheet is provided as a surface layer on these articles that is intended to be placed against the wearer""s skin. Body fluids pass through the apertures in the topsheet into the fluid management and retention layers below.
The topsheet desirably should have a number of properties. Fluids should be rapidly drawn away from the surface of the skin and the skin should be kept dry thereafter and these properties should remain at high levels after multiple insults. The topsheet layer should be pliable, soft, and cloth-like in appearance and touch. The topsheet should also be sufficiently strong to withstand rough-and-tumble wear by the physically active without binding, tearing, or producing excessive lint or broken filaments extending from the surface of the web.
Beyond the desirable properties of the topsheet, the nonwoven web from which the topsheet or other component of a disposable absorbent article is prepared should be sufficiently strong to withstand aperturing and other processing steps. Several of the desirable properties of various nonwoven webs that make good topsheet candidates are somewhat in conflict and are difficult to balance in a single nonwoven web. For example, softer webs made from polyethylene may not be producible at commercially significant rates and may shred during aperturing. The cost of polyethylene is a factor limiting its use. Nonwoven webs used as a topsheet typically have been prepared from lower cost polypropylene, which has a high speed production rate, but produces a web that may be less soft than is desirable and aperturing can result in a higher percentage than desired of broken filaments.
Aperturing can be accomplished by a number of methods, including thermal, mechanical, and hydraulic methods known in the art. Benson et al. U.S. Pat. No. 5,628,097 describes a method for aperturing a nonwoven web by mechanical stretching. A nonwoven web is said to be weakened along a plurality of locations and then incrementally stretched to cause the web to rupture at the weakened locations. A plurality of apertures is created in the nonwoven web coincident with the weakened locations. The weakened locations are created by a roller arrangement that includes a patterned calender roll and a smooth anvil roll. One or both rolls may be heated and the pressure can be adjusted to concurrently weaken and melt-stabilize the web at a plurality of locations. The web is incrementally stretched after passing through the weakening roller arrangement by being passed through a nip formed by an incremental stretching system that uses opposed pressure applicators having complimentary three-dimensional surfaces.
It would be desirable to improve the properties of nonwoven webs for aperturing and to produce webs at high production rates having a better balance of properties for aperturing and for use as topsheet or other apertured components in the construction of disposable absorbent articles.
The invention provides apertured nonwoven webs suitable for use in disposable absorbent products that have an excellent balance of properties that are desirable for aperturing a nonwoven web and properties that are desirable in components of disposable absorbent articles. The apertured webs of the invention have a high proportion of open area and a combination of softness, high strength, low linting, and cloth-like tactile properties that have previously been unrecognized and unavailable. Elongation, bondability, softness, and machine and cross-direction tensile can be achieved at previously unattained levels.
According to one aspect of the present invention, a nonwoven web is provided which can be readily converted to an apertured web by stretching. The web comprises a plurality of multicomponent fibers comprising at least two thermoplastic polymer components arranged in at least first and second separate continuous structured domains. The polymer component of the first domain comprises polyethylene. Prior to stretch aperturing, the web has a peak elongation of at least 100 percent and is characterized by having a plurality of discrete, spaced-apart, frangible bond sites of polymer bonding the fibers to form a coherent extensible nonwoven web. The frangible bond sites are structured and arranged to readily rupture when subjected to tensile stress to form discrete, spaced-apart apertures in the nonwoven fabric.
The discrete, spaced-apart frangible bond sites can be created in the nonwoven web by applying heat and/or pressure or thermo-mechanical energy (e.g. ultrasonic energy) to the web in discrete areas. For example, the web may be passed through a heated calender nip in which one or both rolls has a patterned surface. The application of mechanical, thermal or thermo-mechanical energy deforms the fibers in the web so that the fibers begin to flow together and bond. During this process, the fibers may completely melt so as to be no longer identifiable. Alternatively, one of the structured domains (e.g. the lower-melting polyethylene) may be deformed so that it adheres to the structured domain of an adjacent fiber, while the other (higher-melting) structured domain of the fiber remains substantially intact to provide strength. This process produces frangible bond sites, which are bonded or fused areas of polymer of predetermined geometry. These frangible bond sites are elongated in shape and have an aspect ratio of at least 3:1.
The web can be mechanically stretched by various conventional methods, such as by passing the web through a nip formed by a pair of incremental stretching rollers having a plurality of teeth and a plurality of grooves or by passing the web over a spreader bar, over a bow roll, or through a tenter frame. When the web is mechanically stretched, it is believed that the frangible bond sites become areas of stress concentration within the web. Tensile stress on the web is communicated to the bond sites. Upon application of sufficient tensile stress, concentrated at the bond sites, the frangible bond sites rupture and apertures are formed coincident with the bond sites.
The first polymer domain, which contains polyethylene, is typically a lower melting point, lower modulus polymer component, should provide at least about 20 to 90 percent by weight of the multicomponent fiber. Aperturing the web tends to produce remnants of the bond site polymer at the edges of the apertures. These edge areas tend to become more pronounced and visible as the concentration of the lower modulus polymer is increased. These regions are believed to improve the integrity of the nonwoven web and one of the benefits of the invention is that these regions are not hard and rigid, but are soft.
A specific example of the invention is a stretch apertured nonwoven web of bicomponent continuous spunbond filaments in which these filaments have a sheath of polyethylene and a core of polypropylene in which the sheath comprises at least 20 percent by weight of the filaments and the core is concentric with the sheath.
Thus, a mechanically stretched apertured nonwoven suitable for use as topsheet or in wipes and the like is provided by the invention that has an excellent balance of properties for withstanding the rigors of mechanical stretching and aperturing and also has a balance of softness, strength and other properties desirable in disposable absorbent articles.